1. Field of the Invention
The invention relates to methods of using polylactate release compounds.
2. Description of the Related Art
Compounds which release hydroxy acids slowly over time preferably α-hydroxy acids can serve as a time-release source of lactic acid for biodegradation of chemical compounds in various media, including soils, aquifers, bioreactors, wastestreams, industrial processes, and other systems. The compounds may also be the basis of formulations which provide a time-release source of lactic acid and other materials and compounds which stimulate growth of microbes and facilitate bioremediation. The lactic acid, which is itself a nutrient for microbes, is broken down to form other compounds which provide both additional nutrients and a source of electrons to support the microbial biodegradation of chemical compounds, preferably halogenated hydrocarbons.
Halogenated hydrocarbons are compounds composed of hydrogen and carbon with at least one hydrogen substituted by a halogen atom (e.g. Cl, Br, or F). Halogenated hydrocarbons are used for many purposes, such as solvents, pesticides, and degreasers. Degreasing products have widespread use in several industries, including dry cleaning, microelectronics, and equipment maintenance. Some of the most common halogenated hydrocarbons are methylene chloride, chloroform, carbon tetrachloride, tetrachloroethane (TCA), tetrachloroethene (PCE), trichloroethene (TCE), dichloroethene (DCE), and vinyl chloride (VC). Such compounds are commonly known as “chlorinated hydrocarbons” or “chlorinated solvents.”
Chlorinated hydrocarbons have been widely used for several decades. This use, in addition to improper handling and storage, has led to extensive soil and groundwater contamination, and these solvents are among the most prevalent groundwater contaminants in the United States today. Contamination of groundwater by chlorinated hydrocarbons is an environmental concern because these compounds have known toxic and carcinogenic effects.
One common technique for decontaminating aquifers that is in current use is the pump-and-treat method. As practiced, this method utilizes a series of extraction wells drilled into a contaminated aquifer. Contaminated water is drawn through an extraction well, treated to remove or degrade the contaminant, and then returned to the aquifer through one or more injection wells or discharged to sewers or other points of non-origin. This method can be time consuming and cost-prohibitive.
Recently, attempts have been made to biodegrade chlorinated solvents in-situ using anaerobic bacteria. Some species of anaerobic bacteria used in bioremediation of chlorinated solvents degrade these solvents by reductive dechlorination. This reductive process requires a steady supply of an electron donor such as hydrogen. Some current research supports the proposition that delivery of hydrogen in a slow, steady manner is an effective way to stimulate and maintain organisms that perform reductive dechlorination and reduce competition for ambient hydrogen by other organisms. Several methods have been proposed to supply the hydrogen needed for reductive dechlorination: addition of short chain organic acids or alcohols; addition of sodium benzoate (as disclosed in U.S. Pat. No. 5,277,815); addition of fats and oils; sparging with hydrogen gas (as disclosed in U.S. Pat. No. 5,602,296); and generating hydrogen gas in-situ by electrochemical reactions or electrolysis (also disclosed in U.S. Pat. No. 5,602,296).
All of the previously mentioned methods have serious shortcomings. Addition of short chain organic acids or alcohols as well as the addition of simple organic esters or organic salts such as sodium benzoate have the problem that essentially all of the chemical is released at once in the area and is free to flow away from the contaminated area. Thus, frequent addition of the chosen compound is needed to keep a sufficient concentration of the compound in the contaminated area over time. The constant injection of high volumes of solutions will increase the volume of the system or aquifer and thereby potentially cause further spread of the contamination. Furthermore, unless special measures are taken to deoxygenate the water and solutions which are injected, the level of oxygen in the system or aquifer will rise, thus harming the anaerobic atmosphere which fosters the microbes performing the reduction.
Sparging with hydrogen requires the installation and use of pipes, manifolds, valves, and other equipment and the handling of large quantities of a highly flammable and explosive gas under pressure. Generation of hydrogen gas in-situ by chemical reaction or electrolysis as disclosed in U.S. Pat. No. 5,602,296 is, by those inventors' own admission, experimental in nature and like sparging suffers from the additional limitation in that hydrogen gas has very low solubility in water. Lastly, addition of fats and oils can provide for the slow release of hydrogen, but the method does not provide a mechanism for controlling the amount of hydrogen released. Furthermore, the amount of hydrogen released is very low compared to the weight of fat or oil that must be added.
One of the most effective substrates to provide hydrogen to a biological system is lactic acid. During anaerobic processes the conversion of lactic acid (or lactate salt) to acetic acid (or acetate salt) liberates two moles of dihydrogen (four moles of elemental hydrogen) for each mole of lactic acid or lactate consumed.
Thus the process produces both an electron source (hydrogen) and a nutrient source for bacteria.
A convenient method of delivering lactic acid is in the form of an ester. Esters of lactic acid hydrolyze to produce free lactic acid, or lactate salt, depending on the pH of the solution.
The hydrolysis reaction can be catalyzed by either acid or base, and the alcohol produced can also serve as a nutrient source for surrounding bacteria. The rate of hydrolysis is dependent upon both the pH and the alcohol with which the ester was formed. Although simple esters of lactic acid, such as ethyl lactate, delay the release of free lactic acid into solution, the lactic acid is still released and converted to hydrogen at a very high rate. This rate may be higher than the rate at which bacteria performing reductive dechlorination can consume it, and thus either be wasted or used by other bacteria which compete with the reductive dechlorinators.